1. Field of the Invention
This invention relates generally to a polyaxial screw and coupling apparatus for use with orthopedic fixation systems. More particularly, the present invention relates to a screw for insertion into spinal bone, and a coupling element polyaxially mounted thereto, via a two-piece interlocking coupling element having a socket portion and a threaded compression member, for coupling the screw to an orthopedic implantation structure, such as a rod, therein enhancing the efficacy of the implant assembly by providing freedom of angulation among the rod, screw and coupling element.
2. Description of the Prior Art
The bones and connective tissue of an adult human spinal column consists of more than 20 discrete bones coupled sequentially to one another by a tri-joint complex which consist of an anterior disc and the two posterior facet joints, the anterior discs of adjacent bones being cushioned by cartilage spacers referred to as intervertebral discs. These more than 20 bones are anatomically categorized as being members of one of four classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine, which comprises the top of the spine, up to the base of the skull, includes the first 7 vertebrae. The intermediate 12 bones are the thoracic vertebrae, and connect to the lower spine comprising the 5 lumbar vertebrae. The base of the spine is the sacral bones (including the coccyx). The component bones of the cervical spine are generally smaller than those of the thoracic and lumbar spine. For the purposes of this disclosure, however, the word spine shall refer only to the cervical region.
Referring now to FIGS. 1, 2, and 3, top, side, and posterior views of a vertebral body, a pair of adjacent vertebral bodies, and a sequence of vertebral bodies are shown, respectively. The spinal cord is housed in the central canal 10, protected from the posterior side by a shell of bone called the lamina 12. The lamina 12 includes a rearwardly and downwardly extending portion called the spinous process 16, and laterally extending structures which are referred to as the transverse processes 14. The anterior portion of the spine comprises a set of generally cylindrically shaped bones which are stacked one on top of the other. These portions of the vertebrae are referred to as the vertebral bodies 20, and are each separated from the other by the intervertebral discs 22. The pedicles 24 comprise bone bridges which couple the anterior vertebral body 20 to the corresponding lamina 12.
The spinal column of bones is highly complex in that it includes over twenty bones coupled to one another, housing and protecting critical elements of the nervous system having innumerable peripheral nerves and circulatory bodies in close proximity. In spite of these complexities, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction. Genetic or developmental irregularities, trauma, chronic stress, tumors, and disease, however, can result in spinal pathologies which either limit this range of motion, or which threaten the critical elements of the nervous system housed within the spinal column. A variety of systems have been disclosed in the art which achieve this immobilization by implanting artificial assemblies in or on the spinal column. These assemblies may be classified as anterior, posterior, or lateral implants. As the classifications suggest, lateral and anterior assemblies are coupled to the anterior portion of the spine, which is the sequence of vertebral bodies. Posterior implants generally comprise pairs of rods, which are aligned along the axis which the bones are to be disposed, and which are then attached to the spinal column by either hooks which couple to the lamina or attach to the transverse processes, or by screws which are inserted through the pedicles.
xe2x80x9cRod assembliesxe2x80x9d generally comprise a plurality of such screws which are implanted through the posterior lateral surfaces of the laminae, through the pedicles, and into their respective vertebral bodies. The screws are provided with upper portions which comprise coupling elements, for receiving and securing an elongate rod therethrough. The rod extends along the axis of the spine, coupling to the plurality of screws via their coupling elements. The rigidity of the rod may be utilized to align the spine in conformance with a more desired shape.
It has been identified, however, that a considerable difficulty is associated with inserting screws along a misaligned curvature and simultaneously exactly positioning the coupling elements such that the rod receiving portions thereof are aligned so that the rod can be passed therethrough without distorting the screws. Attempts at achieving proper alignment with fixed screws is understood to require increased operating time, which is known to enhance many complications associated with surgery. Often surgical efforts with such fixed axes devices cannot be achieved, thereby rendering such instrumentation attempts entirely unsucessful.
The art contains a variety of attempts at providing instrumentation which permit a limited freedom with respect to angulation of the screw and the coupling element. These teachings, however, are generally complex, inadequately reliable, and lack long-term durability. These considerable drawbacks associated with prior art systems also include difficulty properly positioned the rod and coupling elements, and the tedious manipulation of the many small parts in the operative environment.
It is, therefore, the principal object of the present invention to provide a pedicle screw and coupling element assembly which provides a polyaxial freedom of implantation angulation with respect to rod reception.
In addition, it is an object of the present invention to provide such an assembly which comprises a reduced number of elements, and which correspondingly provides for expeditious implantation.
Accordingly it is also an object of the present invention to provide an assembly which is reliable, durable, and provides long term fixation support.
Other objects of the present invention not explicitly stated will be set forth and will be more clearly understood in conjunction with the descriptions of the preferred embodiments disclosed hereafter.
The preceding objects of the invention are achieved by the present invention which is a polyaxial locking screw and coupling element for use with rod stabilization and immobilization systems in the spine. More particularly, the polyaxial screw and coupling element assembly of the present invention comprises a bone screw having a head which is curvate in shape, for example semi-spherical, and a two-piece interlocking coupling element mounted thereto. This combination is mounted inside the bottom of an internal channel of a cylindrical body member.
More specifically, with respect to the cylindrical body member, the tubular body comprises a rod receiving channel formed in the upper portion thereof, with a threading formed on the remaining upper elements so that a rod securing nut and/or set screw may be threaded thereon once a rod has been placed in the channel. The body further includes an axial bore which includes extends from the rod receiving channel through to the bottom of the cylinder. The portion of the axial bore which is below the channel forms a receiving chamber, the upper portion thereof having a constant diameter, and the lower portion of the chamber being inwardly tapered. The inner surface of the upper portion of the chamber and/or the inner surface of the portion of the axial bore which is above the chamber may further include a threading.
The two-piece interlocking coupling element comprises and socket portion and a cap portion. The socket portion is designed with an interior semi-spherical volume, so that it may receive the semi-spherical head of a corresponding bone screw. The interior volume of the socket portion is open at both axial ends thereof. The exterior surface of the socket portion, at the bottom thereof, includes a first set of slots which extend upwardly from the opening so that the interior semi-spherical volume may be expanded or contracted by the application of a radial force. In addition, the exterior surface at the bottom is tapered so that it is narrower at the bottom than at a midpoint. This taper is designed to mate with and nest in the tapered lower portion of the socket portion of the axial bore of the body member.
The upper exterior surface of the socket portion comprises a second set of slots, directed axially along the element to the midpoint, such that the upper opening of the socket element may expand and contract in accordance with the application of a radial force thereon. The exterior surface of this upper section of the socket portion is not tapered and is narrower than the widest taper position of the bottom of the socket portion. The upper section, however, does further include an outwardly extending annular lip at the uppermost axial position. This upper section is designed to be inserted into, and joined with, the cap portion of the coupling element.
The cap portion has a generally cylindrical shape, having an open bottom. The open bottom is inwardly tapered, forming an inwardly extending annular lip, so that as the upper end of the socket portion is inserted, its upper slots are narrowed. Once axially inserted beyond this taper, the upper section of the socket portion expands outward over the inwardly extending annular lip. The inwardly extending annular lip engages the outwardly extending lip of the socket portion so as to prevent disengagement of the two pieces. The socket portion is then permitted to slide into the cap portion, until the larger diameter of the tapered lower portion of the socket contacts the entrance of the cap portion.
The exterior surface of the cap portion may be threaded, so that it may engage a threading of the upper portion of the socket portion and/or the inner surface of the axial bore which is above the socket portion. In addition, the top of the cap includes an opening so that a screw driving tool may directly engage the top of the screw.
The assembly of the entire device begins with the joining of the socket portion to the cap portion of the two-piece interlocking coupling element. This is achieved by the slideable interlocking mating of the two elements. Next, the semi-spherical head of the screw is inserted into the socket portion through the lower expandable opening in the taper portion. Once these parts have been assembled the screw and coupling element should be polyaxially rotateable relative to one another. The screw and coupling element are then inserted through the axial bore of the body (which may require the threading the cap portion of the coupling element along the threading on the inner surface of the axial bore and/or the threading of the cap along the threading of the upper portion of the chamber) until the socket portion nests in the tapered lower portion of the axial bore. If the upper portion of the chamber includes a threading it should not extend beyond the point of the initial nesting of the coupling element in the chamber. This is important because the cap portion must be able to move relative to the socket portion.
In this initial position, the top of the cap portion should rest above the bottom of the rod receiving channel so that a rod, when placed therein, seats directly onto the top of the cap. This direct contact provides the downward force necessary to compress the coupling element into the chamber so that the socket portion is compressed in the tapered portion and locks to the head of the screw.
In a preferred variation of this embodiment, the interior surface of the cap portion includes a slight narrowing taper so that as the cap is compressed downward by the rod, the upper slots of the socket portion are also narrowed, further increasing the crush locking effect on the head of the screw.
The implantation of this screw by a surgeon may proceed first by the assembly of the screw into its initial state. The shaft of the screw is then driven into the vertebral bone at the desired angulation. A rod is then introduced into the rod receiving channel, and the body is angulated into the most ideal position for receiving the rod. A nut and/or set screw is then used to secure the rod in the channel, and simultaneously to provide a sufficient downward translational force to cause the socket portion to be driven into the tapered portion of the chamber in the axial bore, and further to cause the cap portion to drive downwardly also (this further compression locking the screw head in the embodiment wherein the sliding of the cap portion toward the socket portion provides an additional compression on the top of the socket portion and therefore onto the head of the screw).
In a preferred variation, the locking nut comprises a cap nut which has a central post which is designed to provide additional structural support to the inner walls of the element at the top thereof, as well as providing a central seating pressure point for locking the rod in the channel. In either variation, the locking nut seats against the rod and prevents it from moving translationally, axially and rotationally.
Multiple screw assemblies are generally necessary to complete the full array of anchoring sites for the rod immobilization system, however, the screw assembly of the present invention is designed to be compatible with alternative rod systems so that, where necessary, the present invention may be employed to rectify the failures of other systems when the surgery may have already begun.